The use of signal remodulation techniques in wavelength division multiplexed passive optical networks, WDM PON, can provide significant advantages in terms of aggregate transmission capacity and cost. A WDM PON using an inverse return-to-zero, IRZ, modulation format in downstream and a return-to-zero, RZ, modulation format in upstream has been reported in M. Presi et al, “A 80 km reach fully passive WDM-PON based on reflective ONUs”, Optics Express, 10 Nov. 2008, page 19043. This WDM-PON provides a trade-off between cost and performance; low cost optics are used (a reflective semi-conductor optical amplifier, RSOA, at the optical network terminal and direct modulated lasers at the optical line terminal) and good resilience to propagation impairments such as optical reflections and noise is achieved. The WDM-PON reported by M. Presi et al operates at symmetric downstream and upstream bit rates. Some WDM-PONs, such as fibre to the home, FTTH, gigabit PON networks typically operate with asymmetric traffic bit rates, with the downstream rate higher than the upstream rate. For example, a downstream channel used for both data download and video multicast and an upstream channel used just for data upload.